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  <meta charset="UTF-8">
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  <title>Document</title>
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<body>
  <div id="app">按钮</div>

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    let app = document.getElementById('app')
    // app.onclick = function() {}

    // function addEvent(type, el, fn) {
    //   if (window.addEventListener) {
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    //   }
    // }

    function addEvent(type, el, fn) {
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        addEvent = function(type, el, fn) {
          el.addEventListener(type, fn, false)
        }
      } else if (window.attachEvent) {
        addEvent = function(type, el, fn) {
          el.attachEvent('on' + type, fn)
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      }
    }

    addEvent('click', app, () => {})
  </script>
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